An attention has been paid to the use of an excimer laser as a light source of reduction projection aligner (hereinafter called a stepper) for manufacturing semiconductor devices. This is because the excimer laser may possibly extend the light exposure limit to be less than 0.5 .mu.m since the wavelength of the excimer laser is short (for example the wavelength of KrF laser is about 248.4 nm), because with the same resolution, the focal depth is greater than a g line or an i line of a mercury lamp conventionally used, because the numerical aperture (NA) of a lens can be made small so that the exposure region can be enlarged and large power can be obtained, and because many other advantages can be expected.
An excimer laser utilized as a light source of the stepper is required to have a narrow bandwidth with a beam width of less than 3 pm as well as a large output power.
A technique of narrowing the bandwidth of the excimer laser beam is known as the injection lock method. In the injection lock method, wavelength selecting element (etalon, diffraction grating, prism, etc.) are disposed in a cavity of an oscillation stage so as to generate a single mode oscillation by limiting the space mode by using a pin hole and to injection synchronize the laser beam in an amplification stage. With this method, however, although a relatively large output power can be obtained, there are such defects that a misshot occurs, that it is difficult to obtain 100% the locking efficiency, and that the spectrum purity degrades. Furthermore, in this method, the output light beam has a high degree of coherency so that when the output light beam is used as a light source of the reduction type projection aligner, a speckle pattern generates. Generally it is considered that the generation of speckle pattern depends upon the number of space transverse modes. When the number of space transverse modes contained in the laser light is small, the speckle pattern becomes easy to generate. Conversely, when the number of the space transverse modes increases, the speckle pattern becomes difficult to generate art. The injection lock method described above is a technique for narrowing the bandwidth by greatly decreasing the number of space transverse modes. Since generation of speckle pattern causes a serious problem, this technique can not be adopted in the reduction type projection aligner.
Another projection technique for narrowing the bandwidth of the excimer layer beam is a technique utilizing a air gap etalon acting as a wavelength selective element. A prior art technique utilizing the air gap etalon was developed by AT & T Bell Laboratory wherein an air gap etalon is disposed between the front mirror and a laser chamber of an excimer laser device so as to narrow the bandwidth of the excimer laser. This system, however, cannot obtain a very narrow spectral bandwidth. In addition there are problems that the power loss is large due to the insertion of the air gap etalon. Further it is impossible to greatly increase the number of the space transverse modes. Furthermore, the air gap etalon has a problem of poor durability.
Accordingly, an excimer laser device has been proposed wherein a relatively high durable diffraction grating is used as the wavelength selective element. However, in the prior art device utilizing the diffraction grating, there is a problem in the manner of utilizing it so that it is impossible to efficiently reduce the bandwidth.
As above described, the prior art excimer laser device have problems in narrowing the bandwidth, output power, the number of the space transverse modes, or durability so that it has been impossible to use the conventional excimer laser devices as the light source of a stepper.
Accordingly, it is an object of this invention to provide a narrow-band oscillation excimer laser device utilizing a diffraction grating as a wavelength selecting element and capable of efficiently reducing the bandwidth.